Field of the Invention
The present invention relates to an automatic transmission for a vehicle. More particularly, the present invention relates to a planetary gear train of an automatic transmission for a vehicle that improves power delivery performance and fuel economy by achieving nine forward speed stages with a minimum number of constituent elements being used and improves silent driving of the vehicle by using an operation point positioned at a low engine speed.
Description of Related Art
Generally, an automatic transmission achieving more speed stages has been developed to enhance fuel economy and optimize drivability. Recent increases in oil prices are triggering stiff competition in enhancing fuel consumption of a vehicle.
Therefore, much research for reducing weight and enhancing fuel economy through downsizing of an engine and for securing drivability and fuel economy through multiple speed stages of automatic transmissions has been developed.
However, in the automatic transmission, as the number of speed stages increase, the number of internal components (particularly, planetary gear sets) increase, and as a result, a length of the transmission increases. This may deteriorate installability and/or power flow efficiency and may increase production cost, and weight.
Accordingly, development of a planetary gear train which may achieve maximum efficiency with a small number of components may be important in order to increase a fuel economy enhancement effect through the multiple speed-stages.
In this aspect, in recent years, 8-speed automatic transmissions tend to be implemented and the research and development of a planetary gear train capable of implementing more speed stages has also been actively conducted.
However, a conventional 8-speed automatic transmission typically includes three to four planetary gear sets and five to six control elements (frictional elements). In this case, since the length of the automatic transmission increases, mountability may be deteriorated.
In recent planetary gear trains, one planetary gear set is disposed above another planetary gear set, but structures of automatic transmissions to which parallel planetary gear sets is applied are very limited.
In other planetary gear trains, dog clutches instead of control elements of wet-type are used. However, shift feel may be deteriorated.
The information disclosed in this Background of the Invention section is only for enhancement of understanding of the general background of the invention and should not be taken as an acknowledgement or any form of suggestion that this information forms the prior art already known to a person skilled in the art.